High performance integrated circuit chips such as microprocessors require large values of bypass capacitance located closely adjacent to the chip. In some recent designs, such as the Digital Equipment Corporation Alpha 600 MHz microprocessor, it has been necessary to wirebond the bypass capacitor directly to the chip. In this particular example, the “bypass capacitor” is itself a specialized P channel FET chip with a large thin oxide region. Capacitor values are typically 10 nanofarads or more. Practical values of inductance range from a fraction of a nanohenry to a few nanohenrys. A simple calculation shows that such circuit combinations lead to resonant frequencies very close to the clock frequency of the microprocessor. The microprocessor clock itself contains many harmonics of the basic clock frequency. Specific repetitive operations of the microprocessor are capable of generating still other harmonics and subharmonics. Thus, it becomes nearly impossible to escape harmonics or subharmonics that can excite the resonant frequency of the bypass capacitor and its ancillary inductance. Adding resistance to the inductance to provide critical damping, destroys the efficacy of the bypass capacitor.
Accordingly, a voltage supply bypass capacitor is needed for high performance integrated circuit chips which can be designed to provide maximum losses at selected frequencies.